The 2.5 is the version I prefer as my reader management software. When I bought a new PC just over a year ago, I had to search the internet for this version and it wasn't easy. For this reason alone, you may be better off with the newer version.
As for it's merits as a proof-reading device: imho the ebook looks totally different on the ereader to how it looks on the software. The best way of seeing how your book will look on the ereader is to purchase a second-hand Sony reader.